The demonstration, the largest since SB 1070 was signed into law in April, draws a diverse crowd. Backers of the law hold their own rally at a suburban stadium in the evening.

Under a broiling desert sun, tens of thousands of protesters on Saturday slowly marched five miles to the state Capitol to rally against Arizona's controversial new immigration law.

There was no official crowd estimate, but the march was by far the biggest demonstration since Gov. Jan Brewer signed SB 1070 into law on April 23. The law makes it a state crime to lack immigration papers and requires police to determine the status of people they stop and suspect are in the country illegally.

The law's backers, who held their own rally at a suburban stadium Saturday evening, contend that the measure is necessary to protect against violence seeping across the border from Mexico. "Why not make the country like it's supposed to be? Borders define us," said Don Baggett, who came to the rally from a Houston suburb.

Critics including President Obama, whose Justice Department is expected to challenge the law in court, say the measure invites racial profiling.

Several groups have sued to stop the law from going into effect as scheduled July 29. It is widely popular in Arizona and has attracted majority support in several national polls.

State Atty. Gen. Terry Goddard, a Democrat who opposed the law and is running for governor, announced Friday that he would defend the state against federal litigation. But Brewer, his likely Republican opponent, said Friday night that she didn't trust Goddard and would select private attorneys to handle the case.

Saturday was dominated by the voices against the law, and the march shut down much of this city's center.

Demonstrators came from as far as Rhode Island and Louisiana. They streamed down the broad boulevards in a several-blocks-long procession of white shirts, American flags and umbrellas to protect against the sun and temperatures in the high 90s. Dennis DuVall, 68, drove down from Prescott, Ariz., 100 miles north.

"It's my civic duty," said DuVall, a retired bus driver. "It shows commitment. People are willing to come out and walk five miles in 100 degrees. It's important."

The diverse crowd included a number of families like the Baezes, who drove from San Diego on Friday night. Juan and Guadalupe Baez, their six children ranging in age from 2 to 18 and Guadalupe's mother all wore matching white T-shirts that Juan, a 43-year-old trucker, had designed. On the back were the words, "We are hard workers, not criminals! We believe in USA justice. Arizona's SB 1070 is not justice."

"It's good to come here to help people," said Baez, who emigrated from Mexico illegally 24 years ago but was legalized in the amnesty signed by former President Reagan in 1986. He is now a U.S. citizen.

Passage of the Arizona law shocked him. "I thought the government is more noble, more fair here," he said.

Another family pushed a toddler who held a hand-lettered placard that said, "Mommy, why is my skin color a crime?" Several banners targeted President Obama, with whom many Latino activists express disappointment for failing to push immigration reform. "Obama ¿Donde Esta la Reforma?" asked one.

Another banner, carried by a group of students, declared, "CSU Bakersfield: We Are Arizona."

Hours later, several thousand supporters of the law filled most of a ballpark in Tempe where the Angels play during spring training. A series of talk radio hosts revved up the crowd while a musician played "Hit the Road Jack," and "tea party" groups signed up new members. One member of a Texas tea party, a legal immigrant from Colombia, spoke in Spanish of her support for the law.

"It's difficult for me to understand why you Americans have to pay for so many people who are not citizens," said Victoria Dennis of Dallas, as her husband, Philip, translated to wild applause. "This has got to stop, not only here in Arizona, but in America."